During the development and production of a hydrocarbon bearing formation, small quantities of well fluids, or sample fluids, are obtained for analysis. Specifically, a sample fluid is extracted from the formation and transferred to laboratory equipment so that physical properties of the sample fluid, such as specific gravity, viscosity and salt water content, for example, may be determined. Of the aforementioned physical properties, the viscosity of a sample fluid, i.e., the sample fluid's resistance to flow or the sample fluid's ratio of shear stress to shear rate, is of particular importance to the development and production of a hydrocarbon bearing formation.
Numerous techniques and pieces of laboratory equipment are available for measuring the viscosity of the sample fluid. Various factors that are considered when determining the appropriate technique and equipment for measuring viscosity include the potential viscosity of the sample fluid, the state of the sample fluid, conditions, i.e., temperature and pressure, at which the measurements are to be made, and the volume of the sample fluid available for the measurements.
A capillary tube viscometer provides a convenient and reliable method for measuring the viscosity for a wide range of sample fluids. In principal, the capillary tube viscometer is an instrument for measuring viscosity of a sample fluid by passing the sample fluid at a known flow rate through a capillary tube which has a known diameter and a known length.
Quite often, the most pressing limitation in determining the viscosity of a sample fluid is the volume of the sample size available. This is particularly true in instances of offshore exploration for hydrocarbons wherein a small volume, such as 20 mL, of the sample fluid is collected for testing. Many viscometers, however, may have a working volume requirement far in excess of 20 mL and often approaching 500 mL.
To overcome the working volume limitations, some existing viscometers utilize mercury as a weighted fluid. Mercury is an incompressible, non-corrosive, non-interacting, well-characterized liquid that can be used to fill the volumetric gap and exert hydraulic pressure on a sample fluid without in any way altering the properties of the sample. Specifically, mercury (Hg) is a silvery liquid metal that is stable with air and water and unreactive to acids, except concentrated nitric acid (HNO3), and alkalis. In addition, mercury is stable across a wide range of temperatures, i.e., mercury is liquid at room temperature and has a melting point of 234.28 K and a boiling point of 629.73 K. The density of mercury is 13,546 K/m3 at 293K which is far greater than the density of the formation sample fluids. The single most potent attribute of mercury is its non-interacting features, i.e., mercury left in contact indefinitely with an aqueous or hydrocarbon system will shown no interaction. Consequently, the properties of the aqueous or hydrocarbon systems do not change, and any measurements made on the fluid phase in contact with mercury can be safely assumed to represent the true properties of the fluid phase.
It has been found, however, that although mercury is a naturally occurring element that is present throughout the environment, human activity can release mercury into the environment that results in population poisonings, high-level exposures of occupational groups and contamination of aquatic-based food chains. In particular, when mercury enters water, biological processes transform it into a highly toxic form, such as methylmercury (CH3Hg), that accumulates in fish and animals that each fish. Human exposure then results by eating the mercury-contaminated fish and animals. Accordingly, in order to protect people's health and the integrity of the environment, in recent years industries have been working to reduce the amount of mercury in the environment by searching for alternative processes and techniques which reduce the likelihood of mercury contamination or avoid the use of mercury.
Therefore, a need has arisen for an apparatus and method for measuring the viscosity of small volume sample fluids that closes the volumetric gap between the sample fluid and the working volume. In addition, a need has arisen for such an apparatus and method that preserve the integrity of the sample fluid. Further, a need has arisen for such an apparatus and method that eliminates the use of mercury.